What is the reason for data retention?

The justifications for accessing retained data are set out in the UK Regulation of Investigatory Powers Act 2000. They are:

• in the interests of national security;
• for the purpose of preventing or detecting crime or of preventing disorder;
  
• in the interests of the economic well-being of the United Kingdom;
  
• in the interests of public safety;
• for the purpose of protecting public health;
• for the purpose of assessing or collecting any tax, duty, levy or other imposition, contribution or charge payable to a government department;
• for the purpose, in an emergency, of preventing death or injury or any damage to a person's physical or mental health, or of mitigating any injury or damage to a person's physical or mental health;
• for any purpose (not falling into the above) which is specified for the purposes of this subsection by an order made by the Secretary of State.